This disclosure relates to a transmission and reception apparatus and a transmission and reception system.
A transmission and reception system wherein an image is transmitted in the form of an analog signal using a coaxial cable or the like is used widely, for example, as a monitoring camera system.
In a transmission and reception system wherein an analog signal is transmitted using a coaxial cable or the like, as the frequency of a signal to be transmitted increases, and as the transmission distance increases, attenuation of the signal increases. Accordingly, transmission of image data or the like in the form of an analog signal is limited in terms of the length of the transmission distance from a point of view of degradation of the image caused by attenuation of the signal.
Therefore, there is a demand to use a digitally modulated signal by use of quadrature amplitude modulation (QAM) or quadrature phase shift keying (QPSK) or the like in transmission and reception systems adopted in a monitoring camera system and other systems as well, so as to achieve transmission of data such as image data which suffers less degradation. In transmission wherein a digitally modulated signal is used, even if the transmission distance is long, if the strength of the signal exceeds a certain threshold level, then an image which is not degraded significantly can be received by the reception side by carrying out digital signal processing such as error correction.
In order to generate a signal digitally modulated by QAM, QPSK or the like, usually a signal based on a carrier, a data rate and a modulation method is first generated by a digital signal processing circuit. The generated signal is inputted to a D/A converter, and thus a digitally modulated signal is obtained.
In general, a D/A converter carries out sampling based on a predetermined clock signal. The D/A converter therefore outputs a carrier to be generated and a folded signal which is the difference between the clock signal and the carrier. Since the folded signal causes folding distortion, usually an anti-aliasing filter is disposed on the output side of the D/A converter to attenuate the folded signal included in the transmission signal. The technique just described is disclosed, for example, on page 3 of a document searched Nov. 24, 2011 on the Internet (URL: http://www.analog.com/static/imported-files/jp/overviews/ADI_Data_Conversion_Poster_F.pdf).